Molecular Simulation Study of Elasticity of Fluid-Saturated Zeolites

Zeolites are widely used for applications involving gas adsorption thanks to their crystalline porous structure, high surface area, and mechanical stability. Experiments using magnetoelastic sensors showed that gas adsorption can noticeably alter the elastic moduli of zeolites. Here we utilized a combination of the classical molecular dynamics and Monte Carlo simulations to explore this effect in silico, calculating the bulk moduli of the zeolites, of the fluids adsorbed in these zeolites, and of the zeolite-fluid composites. We considered two gases -- nitrogen and carbon dioxide, in two zeolite structures -- 13X and 4A. The moduli of the composite systems were calculated using two alternative approaches: from the moduli of the constituents (unsaturated zeolite and adsorbed fluid) using the Gassmann equation, and directly, using the Birch-Murnaghan equation of state. The direct approach showed better agreement with the experimental observations, raising a question on the applicability of the Gassmann equation for microporous zeolites.